Robotic work tools are available on the market in different types. One common type is a robotic lawnmower, other types include robotic vacuum cleaners, robotic floor cleaners, robotic snow removers and robotic garbage collectors. A robotic lawnmower will be used as a non-limiting example of a robotic work tool in this document; a skilled person will however realise that the teachings in this document are applicable also to other types of robotic work tools.
Robotic lawnmowers are extensively used for lawn maintenance activities. Typically, a robotic lawnmower is an autonomous robotic tool which uses an onboard battery as a power source. Based on the operating load and duration, there is a need to recharge the battery of the robotic lawnmower periodically. Since an autonomous robotic lawnmower works unattended, it is required to find a path to a charging station in case the battery power level falls below a threshold power level during operation. There are many techniques which are currently used to guide the robotic lawnmower back to the charging station. For instance, an antenna built on the charging station may be used to navigate the robotic lawnmower to the charging station. However, the antenna may have a limited range in a vicinity of the charging station, and there is a risk that the robotic lawnmower will have difficulties in finding the charging station before the remaining battery power runs out.
Therefore, many robotic lawnmowers are configured to follow a guiding wire that may be used to navigate the robotic lawnmower to the charging station. The guiding wire may be a boundary wire which demarcates, i.e. delimits the perimeter of, an intended work area of the robotic lawnmower. Alternatively or additionally, a shorter straight guide wire or wire loop may be provided in front of the entrance to the charging station, serving to assist the robotic lawnmower to safely approach and dock with the charging station. Both a boundary wire and an entrance guide wire may, in a non-limiting manner, constitute a “guiding wire” for embodiments disclosed in this document.
In order to be able to follow the guiding wire, the robotic lawnmower is provided with one or more sensors adapted to sense the intensity or strength of a magnetic field generated by the guiding wire. The sensor(s) may for instance be vertical loop sensor(s). However, if the robotic lawnmower would follow the same path each time, it could result in undesirable permanent visible tracks and/or markings on the lawn. To overcome the above mentioned disadvantages, robotic lawnmowers may be configured to follow along the guiding wire at a distance. This distance may be selected for instance randomly each time the robotic lawnmower navigates to the charging station. The robotic lawnmower follows fixed magnetic field intensity while traveling back to the charging station. The fixed magnetic field intensity is selected to have a different value from one cycle to another. Thus, the robotic lawnmower takes a different path each time it travels to the charging station. This approach effectively addresses the problem of visible tracks on the lawn.
Generally, longer distances to the guiding wire will yield lower values of strength of the magnetic field detected by the sensor(s). The relation between detected magnetic field strength (i.e. the value of the sensor output signal) and distance to the guiding wire allows the robotic lawnmover to follow along the wire at the desired distance. However, the present inventor has realised certain complications and problems with the typical behavior of the sensor output signal. An exemplifying illustration of the sensor output signal is given in FIG. 4A.
First, the relation is non-linear. Second, the sensor output signal varies more heavily with distance for shorter distances to the guiding wire. Third, the sensor output signal even changes in the opposite direction when the distance to the guiding wire is very short, exhibiting even stronger variations also for very small differences in distance. Fourth, the sensor output signal becomes zero when the robotic work crosses the guiding wire, and then assumes opposite polarity at the other side of the guiding wire.
Because of these complications, prior art robotic lawnmovers have shortcomings in their ability to follow a guiding wire at a broad range of different distances, including distances very close to the guiding wire, and/or distances at both sides of the guiding wire. Many prior art robotic work tools also have short comings when following a guiding wire through a corner. Thus there is a need for improvements in these regards.